The present invention relates to a process of preparing lower vinyl chloride polymer, and in particular to a process of preparing lower vinyl chloride polymers which are useful in producing hard polyvinyl chloride articles useful for applications requiring high moldability and good heat resistance, e.g., bottles, containers, films, and building materials, such as rainwater pipes, window frames, gutters, corrugated sheets or the like.
Vinyl chloride polymer is used in a wide range of applications, since, by adding different kinds of plasticizers to the polymerization mixture and controlling the polymerization conditions, it is possible to obtain vinyl chloride polymers having a wide range of hardness and mechanical properties, chemical resistances, weather resistances, and transparencies.
To produce such hard polyvinyl chloride articles, only lower vinyl chloride polymers having a polymerization degree of no more than 800 can be used to secure the required physical properties.
One industrial method for producing such lower vinyl chloride polymers has been to simply increase the polymerization temperature. If, for example, a polymerization degree of about 600 is desired the polymerization temperature should be 70.degree. C. or higher, assuming all other polymerization conditions are conventional. However, it has been found that at such high temperatures, the primary-stage reaction will be undesirably accelerated, with the result that the polymer produced will be heterogeneous. Furthermore, at such high temperatures, the vapor pressure of the vinyl chloride monomer will rise to as high as 13 kg/cm.sup.2, so that it is necessary to increase the pressure capacity of the polymerization reactor.
In order to produce lower polymers industrially at lower temperatures, methods have been attempted wherein the polymerization is carried out in the presence of so-called "chain transfer agents" such as an organic mercaptan or a halogenated hydrocarbon. However, at a low polymerization degree, especially 700 or lower, using a chain transfer agent often results in an unstable suspension polymerization mixture, and the resulting polymer particles are so coarse that the particle size distribution is abnormally distorted. In extreme cases, the polymer particles coagulate to form blocks. Attempts at increasing the amount of the suspension agent to compensate for this problem resulted in a too wide particle size distribution so that the gelation characteristic becomes nonuniform and the workability becomes poor. A further disadvantage of use of a chain transfer agent is that residual agent causes degradation of the thermal stability of the polymer.